linux-kernel - Re: [PATCH] tracing: Fix wraparound problems in "uptime" tracer

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Date:	Fri, 18 Jul 2014 14:47:33 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tony Luck <tony.luck@...el.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, <fweisbec@...il.com>,
	<m.chehab@...sung.com>, "Xie XiuQi" <xiexiuqi@...wei.com>
Subject: Re: [PATCH] tracing: Fix wraparound problems in "uptime" tracer

On Fri, 18 Jul 2014 11:43:01 -0700
Tony Luck <tony.luck@...el.com> wrote:

> The "uptime" tracer added in:
> 
>     commit 8aacf017b065a805d27467843490c976835eb4a5
>     tracing: Add "uptime" trace clock that uses jiffies
> 
> has wraparound problems when the system has been up more
> than 1 hour 11 minutes and 34 seconds. It converts jiffies
> to nanoseconds using:
>         (u64)jiffies_to_usecs(jiffy) * 1000ULL
> but since jiffies_to_usecs() only returns a 32-bit value, it
> truncates at 2^32 microseconds.  An additional problem on 32-bit
> systems is that the argument is "unsigned long", so fixing the
> return value only helps until 2^32 jiffies (49.7 days on a HZ=1000
> system).
> 
> Avoid these problems by using jiffies_64 as our basis, and
> not converting to nanoseconds (we do convert to clock_t because
> user facing API must not be dependent on internal kernel
> HZ values).
> 

Do you think this is worthy of stable and 3.16 material?

-- Steve

> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  kernel/trace/trace.c       | 2 +-
>  kernel/trace/trace_clock.c | 9 +++++----
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 384ede311717..eb7d56480545 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -809,7 +809,7 @@ static struct {
>  	{ trace_clock_local,	"local",	1 },
>  	{ trace_clock_global,	"global",	1 },
>  	{ trace_clock_counter,	"counter",	0 },
> -	{ trace_clock_jiffies,	"uptime",	1 },
> +	{ trace_clock_jiffies,	"uptime",	0 },
>  	{ trace_clock,		"perf",		1 },
>  	ARCH_TRACE_CLOCKS
>  };
> diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
> index 26dc348332b7..57b67b1f24d1 100644
> --- a/kernel/trace/trace_clock.c
> +++ b/kernel/trace/trace_clock.c
> @@ -59,13 +59,14 @@ u64 notrace trace_clock(void)
>  
>  /*
>   * trace_jiffy_clock(): Simply use jiffies as a clock counter.
> + * Note that this use of jiffies_64 is not completely safe on
> + * 32-bit systems. But the window is tiny, and the effect if
> + * we are affected is that we will have an obviously bogus
> + * timestamp on a trace event - i.e. not life threatening.
>   */
>  u64 notrace trace_clock_jiffies(void)
>  {
> -	u64 jiffy = jiffies - INITIAL_JIFFIES;
> -
> -	/* Return nsecs */
> -	return (u64)jiffies_to_usecs(jiffy) * 1000ULL;
> +	return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
>  }
>  
>  /*

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